metals & alloys

Question 1

what are metals and alloys used for in dentistry?

Answer 1

RPD framework
crowns
denture base
orthodontic appliance
restorations

Question 2

what is the main drawback of metals?

Answer 2

poor aesthetics

Question 3

what is the advantage of metals?

Answer 3

superior mechanical properties

• strength
• rigidity
• hardness
• elastic limit
• some ductility

Question 4

what is a metal?

Answer 4

aggregate of atoms in crystalline structure

Question 5

what is an alloy?

Answer 5

combination of metal atoms in crystalline structure

Question 6

what is ductility?

Answer 6

amount material will deform without fracture - subject to tensile stress

Question 7

what property is demonstrated if compressive stress applied?

Answer 7

malleability

Question 8

what does the crystalline structure of a metal depend on?

Answer 8

history- method of production

shaping- crucial for dental applications eg. cold working

Question 9

which factors affect mechanical properties?

Answer 9

crystalline structure
grain size
grain imperfections

Question 10

what are the 3 lattice arrangements?

Answer 10

cubic, face-centred cubic, body centred cubic

Question 11

how are grains formed?

Answer 11

molten state above melting point
cools
changes state 
plateau- liquid and solid
solid cools

Question 12

describe crystal growth

Answer 12

first atoms act as nuclei of crystallisation
crystals grow to form dendrites
crystals (grains) grow until they impinge on other crystals
region where grains make contact called grain boundary

Question 13

what is it called if crystal growth is of equal dimension in each direction?

Answer 13

equi-fixed grains

Question 14

what is radical grain structure?

Answer 14

molten metal cooled quickly in cylindrical mould

Question 15

what is fibrous grain structure?

Answer 15

wire pulled through die (narrow circular aperture)

-cold worked metal/alloy

Question 16

what is the result of fast cooling (quenching)?

Answer 16

more nuclei

small fine grains

Question 17

what is the result of slow cooling on crystal growth?

Answer 17

few nuclei

large coarse grains

Question 18

what are nucleating agents?

Answer 18

impurities or additives act as foci for crystal growth

Question 19

what is a grain boundary?

Answer 19

change in direction of the crystal planes

Question 20

why are small grains advantageous?

Answer 20

high elastic limit

increased FS and UTS, hardness

Question 21

what is the disadvantage of small grains?

Answer 21

decreased ductility

Question 22

what are the factors needed to quench?

Answer 22

small bulk
heat metal/alloy just above melting point before cooling
store in mould/container that readily conducts heat

Question 23

how do defects affect crystal/grain?

Answer 23

weaken grain so overall structure
grain not perfect
bond less effective than perfect ones
if force applied, defect more likely to be fractured

Question 24

what is the result if force applied to defect?

Answer 24

defect moves along lattice plane

• crystal shape changes
• characteristics of metal change
• until reaches grain boundary so grain has all atoms inter-connected with same strong bond

Question 25

what effect does impeding movement of dislocations have on metals/alloys

Answer 25

``` increases: -elastic limit -fracture stress/UTS -hardness decreases: -ductility -impact resistance ```

Question 26

what factors impede dislocation movement

Answer 26

grain boundaries alloys-dif atom sizes cold working -dislocations stopped at grain boundaries

Question 27

what is cold work?

Answer 27

change shape by bending rolling etc. done at low temp (below recrystallisation temp) causes slip so dislocations collect at grain boundaries so stronger, harder material

Question 28

what does cold work increase?

Answer 28

elastic limit fracture stress/UTS harness residual stress

Question 29

what does cold work decrease?

Answer 29

ductility impact strength corrosion resistance

Question 30

what is the impact of residual stress?

Answer 30

causes instability in lattice results in distortion over time undesirable

Question 31

what is residual stress relieved by?

Answer 31

annealing

Question 32

what is annealing?

Answer 32

heating m/a so that greater thermal vibrations allow migration of atoms re-arrangement allows any instability/distortion w/i lattice to be eliminated

Question 33

when is recrystallisation used?

Answer 33

when cold working hasn't had desired effect?

Question 34

what is recrystallisation?

Answer 34

``` m/a heated causing: -smaller, equiaxed grains -lower EL, UTS, hardness -increased ductility re-set allows further cold work can repeat until correct shape obtained ```

Question 35

what determines recrystallisation temp?

Answer 35

depends on amount of cold work | greater cold work-> lower temp needed

Question 36

what effect does excessive temp rise cause on grain growth?

Answer 36

large grains replace smaller course grains yielding poorer mechanical properties